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Pallasites: Olivine-Metal Textures, Phosphoran Olivine, and Origin

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Pallasites: Olivine-Metal Textures, Phosphoran Olivine, and Origin Pallasites: olivine-metal textures, phosphoranolivine, and origin Ed Scott University of Hawai’i, Honolulu, HI 96822. [email protected] Four questions: 3. Origin of the four FeO-rich MG pallasites 1. Why do some pallasites have rounded olivines? 2. Why is phosphoran olivine only found in five main-group pallasites? 3. Why do four main group pallasites with rounded olivines contain abnormally Fe-rich olivine? 4. Where did main group and Eagle Station group pallasites form? a 1. Origin of rounded olivine Did cm-sized rounded olivines form before angular olivines [9], or were olivines rounded after Fig. 5. Zaisho, Springwater, Rawlinna, and Phillips Co. fragmentation of olivine [2.3,7]? Constraints: contain anomalously Fe-rich olivine, Ni-rich metal, and • Uniform size distribution of rounded olivines in farringtonite Mg3(PO4)2 [20] suggesting Fe was oxidized. Fig.1c is unlike that of angular olivine pallasites Fig. 3. Ir vs. Ni showing metal in main group pallasites and IIIAB irons. Arrows show Data from [2]. which may contain dunite fragments several cm solid and liquid Fe-Ni paths during fractional crystallization. MG pallasites were b in width (Fig. 1a.). mostly formed by mixing of residual metallic liquid from a IIIAB-like core after 75- Springwater contains 4 • Angular olivine pallasites may contain pieces of 80% crystallization [1,2]. Pavlodar (Pa) has rounded olivines and plots near the vol.% farringtonite (center) rounded olivine texture (Fig. 1b). initial melt composition showing that rounding preceded metal crystallization. enclosing olivine. Slice ~12 • Pavlodar metal matches that expected for initial Marjalahti (Ma), Huckitta (Hu) and Seymchan (Se) have angular olivines and cm wide. Oxygen isotopes melt (Fig. 3) showing that rounded olivines formed after ~30-50% crystallization. Br: Brenham, Kr: Krasnojarsk, Ra: Rawlinna, are main-group [18]. D. H. formed before significant metal crystallization. Sp: Springwater, TM: Thiel Mountains. Adapted from [2]. Ball, Arizona State Univ. • Rounding is bimodal: either micro-rounding in c solid metal or macro-rounding in molten metal (Fig. 2). Since Springwater lacks pyroxene, Davis & Olsen [19] Conclusion: Brenham-like texture with cm-sized suggested that farringtonite formed from O and P from metal rounded olivines is the primary texture. and Mg from olivine: 3Mg2SiO4 + 6Fe + 4Pmetal + 8O2 = 2Mg3(PO4)2 + 3Fe2SiO4 Note: Not all rounded olivine, low-Ir MGP have farringtonite. Brenham, which has normal Fa olivine, contains 10 cm wide 2. P-rich olivine chromite that probably formed by oxidation of Cr in molten metal [21]. Fig. 1. Diverse textures in MG pallasites. a) Tiny grains of P-rich olivine and Ca and Ca- Mantle dunite in fragmental olivine matrix in Fe-Mg phosphates formed from trapped Admire. b) Olivine aggregate containing rounded silicate melt that was enriched in P from olivines in fragmental olivine matrix (lower left) in molten metal via redox reactions [12-14]. 4. Origin of MG and Eagle Station pallasites Seymchan. L. Labenne. c) Rounded olivine P-rich olivine is found only in low-Ir pallasites texture in Brenham. Monnig Collection. Widths: with rounded olivine. Figs. 3 & 4 explain why. a, 15 cm; b, 6.4 cm; c, 9 cm. Fig. 4. Cartoon showing how dunite mantle (zone 1) and Brenham-like olivine (zone 3) may form at the core-mantle boundary. Zone 2 is a metal- poor Brenham-like region. Fig. 1a-1c show olivines from zones 1-3, a b respectively. Adapted from Wood [8]. Conclusions 1. Cm-sized rounded olivines formed when equant olivine crystals were immersed in molten metal at the core-mantle boundary. Angular olivine fragments formed later and were micro-rounded in solid Fe,Ni. 2. Phosphoran olivine formed in MG pallasites from trapped pockets of P-rich silicate Fig. 6. △17O vs. ɛ54Cr showing mass-independent isotopic melt [12-14]. It occurs in pallasites with rounded olivines and low-Ir metal (≤0.1 ug/g) variations in meteorites and planets. Eagle Station group as these contained trapped silicate melt that equilibrated with molten P-rich, Ir-poor pallasites plot with carbonaceous chondrites in the lower metal. right; main group pallasites plot on the left with the Fig. 2. Micro-rounding of angular cm-sized fragments of olivine in Admire. 3. Fe-rich olivine formed in four MG pallasites when Fe was oxidized to replace Mg terrestrial planets, OCs, ECs, and most achondrites and Green box near center of (a) shows location of magnified image in (b), which from olivine that combined with P and O in molten metal to form farringtonite, shows mm-sized olivines are rounded like the cm-sized olivines in Brenham irons [23]. See also Kruijer et al. (this meeting). When Mg3(PO4)2. (Fig. 1c). Experiments show that rounding of cm-sized olivines occurred in Jupiter migrated inwards according to the Grand Tack 4. Eagle Station group pallasites have mass-independent isotopic compositions like model [24] the two populations were intermixed in the molten metallic core [6]. Micro-rounding occurred after solidification. those in carbonaceous chondrites and formed beyond Jupiter. Main-group pallasites asteroid belt. Data from [25], Sanborn, Yin, Irving, formed in the asteroid belt. Both formed from core-mantle boundary materials. Goodrich, Jacquet, Warren, Barrat, Göpel, Schmitz et al. References: [1] Sc ott E.R .D . ( 1977) Min. Mag. 41, 265-272.[2] W as son J .T. and C hoi B.-G. (2003) GCA57, 3079-30 9 6. [3 ] Sc o tt E.R .D . ( 1 9 7 7 ) GCA41, 693-710. [4] Scott E.R.D. et al. (2010) PSR D ( s ee abstr act) [5] Boes enber gJ.S. et al. (2012) GCA89, 134-15 8 . [6 ] Solfer ino G.F.D. et al. (2015) GCA162, 259-275. [7] Ta r d u n o J.A. et al. (2012) Sc ienc e338, 939-94 2 . [8 ] W o od J .A. ( 1 9 78 ) LPS9, 1273-12 7 5; ( 1 9 81 ) LPS12, 1200-1202. [9] Scott E.R.D. and Taylor G.J. (1990) 21, 1119-1120. [10] Yang J. et al. (2010) GCA74, 4471-4492. [11] Bus ec k P. R . ( 1 9 7 7) GCA41, 711-740. [12] Boes enbergJ.S. and HewinsR.H. (2010) GCA74, 1923-1941. [13] Mc Kibbin S. J . et al. (2016) Geochem. Per sp. Let. 2, 68-77. [14] Fowler-Gerace N.A. a n d Ta i t K.T. ( 2015) Amer Min. 100, 2043-20 52 . [1 5 ] Mc Kibben et al. (2013) GCA 119, 1-17. [16] Larsen K.K. et al. (2016) GCA176, 295-315. [17] As phaug E. et al. ( 2006) Nature 439, 155-160. [18] Greenwood R.C. et al. (2015) GCA169, 115-136. [19] Davis A.M and Olsen E.J. (1989) LPS20, 220-221. [20] Bus eck P. R . a n d Holdsworth E. ( 1976) Min. Mag. 41, 91-10 2 . [2 1 ] W as so n J .T. e t a l . ( 1 9 9 9 ) GCA63, 1219-12 3 2. [2 3 ] W a r re n P.W . ( 2 0 11) EPSL 3 1 1, 9 3 -10 0 . [2 4 ] W al s h K.J . e t a l . ( 2 0 11 ) N a tu re 4 7 5, 2 0 6 -209. [25] DauphasN. and Sc haubleE. A. ( 2016) Ann. R ev. Ear th Planet. Sc i. 44, 709-783..
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